1. Field of the Invention
This invention relates to a frequency mixing circuit having an automatic gain control (AGC) function.
2. Description of the Prior Art
In a general radio unit, a frequency mixing circuit capable of controlling a gain automatically is used and, for example, a circuit as shown in FIG. 1 is conventionally known to be used for this purpose. FIG. 1 shows a frequency mixing circuit using a dual gate field effect transistor (FET) Q22, an RF amplifier disposed in the preceding stage thereof and using an FET Q21, and a local oscillator using a bipolar transistor Q23. An RF signal VRF inputted from an antenna is amplified through the RF amplifier and mixed through the frequency mixing circuit with a signal VLO supplied from the local oscillator and outputted as an IF output signal IFOUT. In FIG. 1, L1 to L4 are coils, T1 is an output transformer, C1 to C15 are capacitors, CV1 to CV3 are variable capacitors, C1A, C1B and C1C are ganged variable capacitors, VSS is a source voltage and VAGC is an AGC voltage.
In the conventional frequency mixing circuit as above, an output signal VRF of the RF amplifier is applied to one gate of the FET Q22 and the output signal VLO of the local oscillator is applied to the other gate thereof. The one gate of the FET Q22, namely, the gate applied with the output signal of the RF amplifier is additionally applied through a resistor R5 with a voltage obtained by dividing a power source voltage (-VSS) through a resistor R6 and a resistor RI0. This is to set the gate voltage fixedly by an optimized direct current bias voltage thereby to control the conversion gain of the FET Q22.
In general, the gain of an FET is determined dependently on a transconductance gm and in turn, the transconductance gm is determined dependently on the gate voltage. Similarly, the transconductance gm of a dual gate FET is determined dependently on the voltages to be applied to the two gates thereof, which is monotonically changed with each of such gate voltages. As a result, the frequency mixing circuit in FIG. 1 makes it possible to provide an AGC function because the conversion gain can be changed with the voltage applied to one of the gates of the dual gate FET Q22.
With the frequency mixing circuit as shown in FIG. 1, the AGC function is realized by applying an optimized direct current bias voltage fixedly, thus raising a problem that the controllable range will be narrow. Then, as a modified example, such a frequency mixing circuit that the resistor R1 of the RF amplifier is disconnected from the power source voltage (-VSS) thereby to apply the AGC voltage VAGC to the open end of the resistor R1 as shown by the broken line in FIG. 1 is known as well. In this circuit, the level of the input signal VRF to the FET Q22 can be made variable with the AGC voltage VAGC and as a result, the level of the output signal of the FET Q22 of the frequency mixing circuit, namely, the level of an intermediate frequency output signal IFOUT will be made variable. In this case, however, a problem will arise such that the range where the AGC function is possibly performed is not enough.
In the frequency mixing circuit as shown in FIG. 1, in order to widen the range to perform the AGC function by controlling the gate voltage of the dual gate FET Q22, it is required that the voltage divider consisting of the resistors R6 and R10 is disconnected with the resistor R5 and the AGC voltage VAGC is applied to the open end of the resistor R5 as shown by the broken line in FIG. 1. In this case, however, two input signals are required thereby to make the operation complex and as a result, the operation will be made unstable because the intermodulation characteristic is changed from the optimum bias point when the AGC voltage is applied. Accordingly, there arises such a problem that it is difficult to realize a circuit for performing the AGC function by controlling the gate voltage of the dual gate FET Q22.